1) Field of the Invention
This invention relates generally to fabrication of semiconductor devices and more particularly to a method for forming a copper damascene structure over a tungsten plug with improved adhesion, oxidation resistance and diffusion barrier properties by using nitridation of the tungsten plug.
2) Description of the Prior Art
The term Damascene is derived from a form of inlaid metal jewelery first seen in the city of Damascus. In the context of integrated circuits it implies a patterned layer imbedded on and in another layer such that the top surfaces of the two layers are coplanar.
As device dimensions shrink, the electrical conductivity of the conductive materials used for interconnections (or wiring) becomes increasingly important. Because of its excellent conductivity, copper has become the most attractive interconnect material. Damascene processes have become a common method of forming a copper line contacting underlying tungsten plugs (W-plugs).
However, the top surface of the W-plugs can be oxidized during an etching process to form a trench for a damascene structure, or even from exposure to air. Another problem with current damascene processes is that the oxidized tungsten or tungsten at the top of the W-plugs provide poor adhesion to other materials such as a TaN barrier layer. Also, the tungsten provides a poor diffusion barrier to copper from the damascene structure, allowing the xopper to diffuse into the W-plugs.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering the following patents.
U.S. Pat. No. 5,744,376 (Chan et al.) teaches a Copper plug with a barrier layer.
U.S. Pat. No. 5,314,843 (Yu et al.) shows a WN.sub.x layer on a W layer before a CMP where the WNx layer is used as a CMP-resist layer.
U.S. Pat. No. 5,084,412 (Nakasaki) shows a W-plug covered by a chromium layer and a copper line. The copper line is covered with a nitride metal.
U.S. Pat. No. 5,691,235 (Meikle et al.) shows a method of depositing W-nitride.